Implanted medical devices need a reliable, secure and low-energy wireless communication link. Ultrasound (US) wave propagation is promising over other techniques due to its lower body attenuation, inherent security and well-studied physiological impact. While US communication systems have been proposed, they either neglect realistic channel conditions or fail to be integrated into small-scale, energy-scarce systems. Therefore, this work proposes a custom, hardware efficient OFDM modem optimized for the diverse needs of ultrasound in-body communication channels. This custom OFDM modem is implemented in an end-to-end dual ASIC transceiver: a 180 nm BCD analog front end and a digital baseband chip in 65 nm CMOS technology. Furthermore, the ASIC solution provides tuning knobs to increase the analog dynamic range, to update the OFDM parameters and to fully reprogram the baseband processing, necessary to adjust for the channel variability. Ex-vivo communication experiments on a 14 cm thick piece of beef achieve 470 kbps with BER 3e-4, while consuming 56 nJ/bit and 10.9 nJ/bit Tx/Rx.